On that first run, why does your torque drop off significantly 2000 to 2400? TC lockup issue? Anyways, I'd say get something that stalls in the 2500 area, cause I'm sure your making torque before that the run just hadn't started yet and you usually want to go a few hundred RPM below where your peak torque comes in, so I'd say 2200 to 2500. What torque converter is in there now, if you can somehow find out what the stall is on a factory 5.0 4R70W converter I can drop you a brand new one for cheap. Otherwise another cheap option would be these AODE converters on Ebay by The Edge (I believe that's the name.) Or you'll be stuck buying a $500 PI.
1991 Sport 5.0
4R70W Converted, U4P0/SCT Controlled
, 1.948 60' - 3450 lbs